1. Field of the Invention
The present invention relates to a method of manufacturing a multi-layer printed wiring board.
2. Description of the related art
In recent years, with diversification of users, the types of the printed wiring boards increase, and the life cycles of the printed wiring boards are shortened. For this reason, the manufacturing mode of the printed wiring boards shifts from a conventional inflexible mass production mode to a flexible manufacturing mode.
The printed wiring boards manufactured by a flexible manufacturing system, for example, are used as circuit boards on which circuits for computers (e.g., host computers, computers controlling a security system for dams or buildings) electronic switching systems (digital switching systems: e.g., switching system used in a direct center (DC) or a regional center (RC)), a wireless device (e.g., compact portable telephone), an optical transmission device (e.g., optical submarine cable repeater), PCM transmission system or the like, are mounted. Generally, one to five piece of printed wiring boards of one type, or, at most, ten piece of printed wiring boards of one type are manufactured in once-manufacturing.
The processes in manufacturing a multi-layer printed wiring board comprise an internal layer circuit forming process, an outer layer circuit forming process, a solder resist (also called a “permanent resist”) forming process, and a marking (character printing) process.
Conventionally, the internal layer circuit forming process and the outer layer circuit forming process, for example, an art work film which is formed to remain a conductive pattern as a positive pattern is prepared in advance. Thereafter, the art work film is placed on the surface of a board (substrate) in which a patterning material is covered with a photosensitive film which is photo-hardened, and is irradiated with ultraviolet rays. Thereafter, the art work film is removed, the photosensitive film of a negative pattern portion (portion which is not irradiated with ultraviolet rays) is removed to form an etching resist is formed on the board. Thereafter, the patterning material of the negative pattern portion is etched according to the etching resist, and the photosensitive film of a positive pattern portion (portion which is hardened with ultraviolet-rays-irradiation) is removed. In this manner, an internal layer circuit and an outer layer circuit are formed.
In the solder resist forming process, for example, an art work film formed to remain a positive pattern according to a solder resist pattern is prepared in advance. Thereafter, a solder resist material is coated on the surface of a board on which an outer layer circuit is formed, and the art work film is placed on the solder resist material and irradiated with ultraviolet rays. Thereafter, the art work film is removed, and the solder resist material of the negative pattern portion is removed. In this manner, a solder resist is formed.
In the marking process, a screen plate for a predetermined mark is prepared in advance. By the screen printing using the screen plate, the predetermined mark is printed on a board.
In the method of forming an etching resist (photoresist) or a solder resist by using the art work film (and screen plate), the art work films corresponding to the respective processes must be prepared depending on circuit patterns to be formed. For this reason, in a state of a flexible manufacturing system, the manufacturing cost of the art work film disadvantageously increases.
In addition, the art work film is reserved to be used again. However, when the art work film is distorted or expands and contracts, the art work film cannot be used. For this reason, the keeping is cumbersome and requires a great cost. In particular, in the state of a flexible manufacturing system, an art work film which is rarely used may be keep, and the cost required for keeping may be posed as a problem.
Therefore, a method of forming an etching resist or a solder resist by excluding use of an art work film is desired. Conventionally, the arts in which use of art work films are excluded are as follows:                (1) a method of irradiating ultraviolet rays on a solder resist coated on a substrate in the solder resist forming step (see Japanese Patent Application Laid-Open Publication No. 6-97635);        (2) a method of irradiating a laser beam on a solder resist coated on a substrate in the solder resist forming step (see Japanese Patent Application Laid-Open Publication No. 6-97635);        (3) a method of injecting and coating a solder resist ink on a substrate by an inkjet printer in the solder resist forming step (see Japanese Patent Application Laid-Open Publication No. 6-97635 and 7-263845);        (4) a method of irradiating ultraviolet rays or a laser beam on a marking ink coated on a substrate in the marking step (see Japanese Patent Application Laid-Open Publication No. 6-97635);        (5) a method of injecting and coating a marking ink on a substrate by an inkjet printer in the marking step (see Japanese Patent Application Laid-Open Publication No. 6-97635);        (6) a method of forming a via hole by exposing a photosensitive resin on a substrate by using an exposure laser in the circuit forming step (see Japanese Patent Application. Laid-Open Publication No. 10-48835);        (7) a method of forming an etching resist by the step in which patterning exposure is directly performed by a vector scheme using an electronic beam in the circuit forming step (see Japanese Patent Application Laid-Open Publication No. 6-2198190); and        (8) a method of forming a plating resist by the step in which patterning exposure is directly performed by a vector scheme using an electronic beam in the circuit forming step (see Japanese Patent Application Laid-Open Publication No. 63-190).        
However, the above prior arts have the following problems.
As the first problem, the art (2) described above is a technique in which a spot laser is directly irradiated on a solder resist material to expose the solder resist material. At present, as the solder resist material, an epoxy resin is mainly used. Ultraviolet rays are used in an exposure process for the epoxy resin. For this reason, an ultraviolet laser is used as the spot laser. However, at present, an output of the ultraviolet laser is not sufficient to directly irradiate the ultraviolet laser beam on the solder resist material to expose the solder resist material.
For this reason, time required to directly expose a solder resist material by using an ultraviolet laser is too long. In the art (2), a laser output is increased by focusing a laser beam. However, the laser output of a laser irradiation device itself does not increase, and an area in which the exposure process can be performed decreases per unit time because an irradiation area of the laser beam is decreased by focusing the laser beam. For this reason, time required for the exposure process does not change regardless of the focus of the laser beam.
In this manner, at present, the exposure process for the solder resist material by direct irradiation of a laser beam can be technically realized. However, the exposure process requires time which is considerably longer than that required when an exposure process is performed by using an art work film. For this reason, the exposure process using direct irradiation of a laser beam is not preferable in manufacturing efficiency.
As the second problem, each of the arts (1) to (8) is made to use an art work film for a certain step, and has no point of view for forming an etching resist or a solder resist (sharing a resist forming method) by using the same device and the same material in the internal layer circuit forming process, the outer layer circuit forming process, and the solder resist forming process. For this reason, the number of devices and the number of materials related to the processes in manufacturing a multi-layer printed wiring board increase.